Field of the Invention
The present invention relates to a plastic optical element used in the optical system of an image forming device such as a laser copier, laser printer, facsimile machine, plotter, or complex machine with two or more functions. It also relates to an optical scanner including such a plastic optical element and an image forming device including such an optical scanner.
Description of the Related Art
To adapt for high-speed image outputs, a tandem-type, electrophotographic full-color image forming device in which four photoreceptor drums are arranged with a certain interval has been popular.
In such a tandem type image forming device, the light beams from light sources of an optical scanner or a laser write unit are deflected by a single optical deflector to concurrently expose the surfaces of photoreceptor drums via respective optical scan systems and generate latent images thereon. The latent images are visualized by develop units containing four colors of toner and the four color toner images are transferred onto a paper to form a full color image.
Recently, there have been demands for downsizing and cost reduction of such a tandem type image forming device and an optical scanner. For example, Japanese Patent Application Publication No. 2008-15139 discloses an optical scanner configured to make two light beams incident obliquely on the surface of the optical deflector vertical to the rotation axis and exit from a single optical system to converge on the surfaces of two photoreceptor drums.
Thus, providing only two optical systems for four photoreceptor drums can contribute to reducing the size of the optical scanner.
The optical scanner in the above document includes a combined optical element having two optical effective portions and one optical ineffective portion between the optical effective portions not to allow light transmission. The optical effective and ineffective portions are set such that a distance from a line connecting the apexes of the two optical effective portions to the apex of the optical ineffective portion on the sub scan cross section is to be 1.0 mm or less.
A plastic optical element having two optical effective portions and an optical ineffective portion therebetween can be manufactured by injection molding of melted resin into a mold cavity. During the cooling process of the injection molding, the surface of the plastic optical element is likely to be pulled toward the center with a relatively high temperature due to resin contraction.
A part of the optical effective portions near the optical ineffective portion in sub scan direction is located closer to the center having a high temperature than the rest of the portions. Thus, this part of the optical effective portions is largely affected by thermal contraction of the center area. Because of this, a large local contraction occurs therein, which may cause degradation in mold transferability.
FIG. 15A schematically shows the side surface of a plastic optical element 100 in sub scan direction. The plastic optical element includes two optical effective portions 101, 102 and an optical ineffective portion 103 therebetween on an exit side. FIG. 15B is a graph showing a result of measuring transferability error in the position of the optical surface in sub scan direction. The transferability error refers to a difference between the surface shapes of a mold and a molded product. In FIG. 15B the abscissa axis shows the position of the optical surface in sub scan direction on the exit side (mm) while the longitudinal axis shows transferability error (mm). The position zero is the center of the optical ineffective portion 103.
In FIGS. 15A, 15B, a shift in the position of the optical surface from a mold to a molded product from the incidence side to the exit side is defined to be positive. FIG. 15B shows that resin contraction increases as the shift goes negative in the longitudinal axis. The transferability error is negatively large around the optical ineffective portion 103, that is, the area D in FIG. 15B. It is confirmed that the contraction amount of about the portion 103 is larger than the rest of the surface.
FIG. 16A schematically shows the side surface of a plastic optical element 100a in sub scan direction. The plastic optical element includes two optical effective portions 101a, 102a and an optical ineffective portion 103a therebetween on an incidence side. FIG. 16B is a graph showing a result of measuring transferability error in the position of the optical surface in sub scan direction. In FIG. 16B the abscissa axis shows the position in sub scan direction (mm) on the incidence side while the longitudinal axis shows transferability error (mm). The position zero is the center of the optical ineffective portion 103a. 
In FIGS. 16A, 16B, a shift in the position of the optical surface from a mold to a molded product from the incidence side to the exit side is defined to be positive. FIG. 16B shows that resin contraction increases as the shift goes positive in the longitudinal axis. The transferability error is negatively large around the optical ineffective portion 103a, that is, the area D in FIG. 16B. It is confirmed that the contraction amount of about the portion 103a is larger than the rest of the surface.
In FIGS. 15B, 16B at the transferability error being zero, the element surface is in an ideal state.
To further downsize the optical scanner, the length of the optical ineffective portion of the plastic optical element in sub scan direction needs to be shortened. However, there is a problem that the optical characteristics of a potion of the optical effective portions near the optical ineffective portion deteriorates due to a decrease in mold transferability, which leads to a degradation in beam spot shape on a target scan plane.